Potential research subjects will be identified by a member of the patient's treatment team, the protocol investigator, or research team at MSK or collaborating centers. If the investigator is a member of the treatment team, s/he will screen their patient's medical records for suitable research study participation and discuss the study and their potential for enrolling in the research study. Potential subjects contacted by their treating physician will be referred to the investigator/research staff of the study.

Inclusion Criteria:

High Genetic Risk Cohorts: Participants at high genetic risk for the development of gastric cancer may qualify for either the Early Onset Cancer group, Familial Gastric Cancer group, or the High Risk Relative group. Inclusion criteria for each group is defined below:

•Early Onset Gastric Cancer Patient:

• must have had a diagnosis of gastric or gastroesophageal junction adenocarcinoma made before the age of 50, AND
• have no family history of gastric/ gastroesophageal junction adenocarcinoma (defined as one or more first-degree relatives or two or more second-degree relatives).

These patients will be asked to sign a "Patient Consent Form"

•Familial Gastric Cancer Patient:

• diagnosed with gastric or gastroesophageal junction adenocarcinoma at any age, AND must have one or both of the following:
• One first-degree relative (parents, siblings,offspring) with gastric/ gastroesophageal junction adenocarcinoma
• Two or more second-degree relatives (grandparents, aunts, uncles, niece, nephew, or half-sibling) with gastric/ gastroesophageal junction adenocarcinoma Anybody with a known CDH1 mutation in their family is eligible for the Familial Gastric Cancer patient cohort.

Note, participants who present to the registry with a documented CDH1 mutation within their kindred need not have a diagnosis of gastric/ gastroesophageal junction to enroll within the FGC patient cohort. Moreover, the degree of relation does not affect study eligibility.

These patients will be asked to sign a "Patient Consent Form"

Patients with both early age at onset and family history will be categorized and analyzed as part of the FGC group. Thus, a 30-year old patient with an affected first degree relative would be categorized as "Familial" for the purposes of this protocol even though they also have age of onset before age 50.

When High Genetic Risk gastric cancer patients are identified, we will ask for permission to confirm the diagnosis in their affected relatives by obtaining pathology reports and will ask them to contact their first-degree relatives for potential participation in the study.

High Risk relatives:

• Must be >18 years old AND
• do not have gastric cancer AND
• have one first degree relative who meets eligibility criteria for the EOGC or FGC cohort.

These patients will be asked to sign a "Relative consent form"

Low Genetic Risk Cohorts: Participants at low genetic risk for the development of gastric cancer may qualify for either the Sporadic Gastric Cancer or Non Cancer Control. Inclusion for each group is defined below:

•Sporadic Gastric Cancer patients:

• must have had a diagnosis of gastric/ gastroesophageal junction adenocarcinoma at age of 50 or older AND
• have no family history of gastric/ gastroesophageal junction adenocarcinoma (defined as one or more first-degree relatives or two or more second-degree relatives)

These patients will be asked to sign a "Sporadic Gastric Cancer consent form"

•Non Cancer Control :

• must be >18 years old AND
• do not have cancer AND
• do not have a family history of gastric cancer (defined as one or more first-degree relatives or two or more second-degree relatives)

These patients will be asked to sign a "Non Cancer Control consent form"

At select sites, relatives of participants eligible for the High Genetic Risk cohorts will be invited to undergo a baseline EGD to examine the prevalence of mucosal pathology if they meet the criteria in the protocol AND:

• Are age 35 or older
• Are between 21-35 years old with a relative diagnosed with gastric cancer before age 35. Invitation to undergo a baseline EGD will commence 5-years prior to the age at diagnosis of the youngest affected family member. For example, an individual with a sibling diagnosed with gastric cancer at age 30, will be offered participation in the EGD study commencing at age 25
• Are free of any comorbid illness that would make EGD associated with unusually high risk or would ordinarily obviate them from participation in routine cancer screening (e.g. diagnosis of any medical condition with a poor prognosis).

Exclusion Criteria:

• Have any condition, which in the opinion of the primary MSKCC clinician or investigators precludes their ability to provide informed consent.
• Relatives of patients eligible for the High Genetic Risk Cohorts who are less than 18 years of age are excluded.
• Non- cancer control participants who are less than 18 years of age are excluded.